1. Field of Invention
The present invention is related to implantable cardiac devices such as pacemakers and defibrillators that deliver cardiac resynchronization therapy (CRT), and to a method of programming devices using ultrasound or other techniques such as transthoracic or intracardiac impedance measurements to determine specific parameters indicative of the response of a patient to variations in cardiac stimulation patterns. The parameters are then used to direct closed loop programming of interval timing within such CRT device.
2. Description of the Prior Art
Impaired cardiac performance can result from several abnormalities. Such abnormalities include alterations in the normal electrical conduction patterns and mechanical abnormalities in myocardial contractility. These abnormalities are often (though not necessarily) connected to one another and, as such, electromechanical impairments can cause an impairment in cardiac performance as well. Such impairment in cardiac performance often stems from premature or delayed electrical and/or mechanical events in different cardiac chambers and within specific cardiac chambers. Newly developed cardiac resynchronization therapy devices have been developed as to correct this problem. Unfortunately, such devices do not improve a significant percentage of patients. This is a result of a general inability of such CRT to appropriately correct dysynchronous properties in a customized fashion for each particular patient. There currently are no control systems developed that can provide a tailored approach for resynchronization in individual patients.
Conduction abnormalities may occur between the atria and the ventricular chambers, atrial-ventricular dysynchrony. Abnormalities between right and left ventricular chambers (inter-ventricular) or within the right or left ventricles (intra-ventricular) can result in dysynchrony as well. Dysynchrony leads to ineffective work as a result of forces being generated in specific regions at inappropriate times relative to the opening and closing of the heart valves. It can lead to myocardial relaxation during times where the generation of force in all myocardial segments should be occurring synchronously and in a symmetric fashion in relation to valvular events and myocardial thickening when all myocardial segments should be relaxing, diastole, and receiving oxygenated blood from the lungs. Multiple variations in the location and pattern of dysynchrony may exist in individual patients.
The current understanding of electromechanical dysynchrony is in a state of evolution. Whereas it was once thought that the prolongation of electrical signals as demonstrated by a surface EKG was a specific indication of dysynchrony, more recent data supports that this is not necessarily accurate. Newer ultrasonic imaging modalities such as color Doppler myocardial imaging (CDMI) that quantify myocardial velocity and strain allow for qualification and quantification of myocardial dysynchrony. CDMI is more accurate for tracking synchrony and symmetry of cyclical cardiac events than any other imaging modality and offers the clinician the ability to appropriately program interval timing between stimuli applied by multiple electrodes in CRT devices best suited for an individual patient. However, CDMI does not provide any guidelines of how these timing intervals should be selected, and therefore the process of programming these intervals involves an effort based on trial and error and can be cumbersome and timely. Programming of appropriate interval timing will necessitate experienced physicians who are sub-specialized in the fields of electrophysiology and echocardiography. This will be difficult from a logistic standpoint especially at lower volume institutions or non-academic centers.